Solar Energv "vol. 2f,. pp. 259-263, 1981 0O384)92X/0302594)5502.00/0 Printed in Great Britain. Pergamon Press ltd.

THE OPTICAL CHARACTERISTICS OF SWIMMING POOL COVERS USED FOR DIRECT

SOLAR HEATING

J. L. A. FRANCEY and P. GOLDING Department of Physics, Monash University, Victoria, Australia 3168

(Received 22 February 1980, revision accepted 7 October 1980)

Abstract--Heating of swimming pools by direct absorption of solar radiation is discussed together with the role of pool covers in minimizing re-radiation of the collected energy. Properly designed pool covers behave as selective transmitters, admitting shorter wavelength radiation while being reflective at long wavelengths. Results of optical tests on a number of commercially available pool covers are presented.

I. INTRODUCTION

Solar radiation can play a large part in establishing comfortable conditions in outdoor swimming pools, par- ticuhlrly in warm and temperate climate zones, and pool covers can help to maintain these conditions. The covered pool itself turns out to be a good solar collector and :it is of considerable interest to investigate its pro- perties.

2. ABSORPTION OF SUNLIGHT IN WATER

The direct heating of outdoor pools by solar radiation involves a number of factors which have been reported on by different authors. Surface reflectance has been discussed by several authors [1-6]. The radiation entering water is dispersed and differentially absorbed as dis- cussed by Schmidt[7] and Birge and Juday[8]. From these the spectral distribution of the absorption of sun- light in different depths of swimming pools can be cal- culated and tabulated as in Table 1. The results here are in good agreement with James and Birge's values[9] for natural waters.

Radiation which is not absorbed will strike the bottom and side walls of the pool. Highly absorbing containing walls would allow this radiation to be usefully collected. However, safety considerations usually results in pool wall surfaces being good reflectors so that most of the scattered radiation is returned to the environment. Only a small part of the scattered radiation will be absorbed in the return passage through the water, Table I shows that only a further 5 per cent of the radiation is absorbed in the second metre of water passage.

3. THERMAL RADIATION FROM THE POOL

Heat absorbed in daytime can be lost at night as thermal longwave radiation. Of course heat loss by evaporation and convection also occur. Pool covers have been suggested by Lff[10], Czarnecki [l l ] and others[12, 13] as a means of retarding heat losses but whether a cover will admit solar radiation to the pool while retarding outgoing thermal radiation will depend on its transmission characteristics. These have been measured here for a number of commercially available

Table 1. Absor ~tion characteristics of solar radiation in water

Centre Wave- length (L,m)

<0.38 (ultra- violet)

0.40 (violet)

0.46 (blue)

0.52 (Green)

0 . 4 8 (Yellow)

0.62 (Orange)

0.68 (Red)

>0.68 (Infra- red)

Total Solar S~ectrum

Fraction (%) of Solar Radiation

Fraction (%) of Transmitted Radiation Absorbed in z

Fraction (%) o f Incident Solar Radiation Entering

Water (a) Summer (b) Winter

7.00 (a) 6.6 (b) 6,3

5.47 (a) 5.1 (b) 4.9

8.68 ( a ) 8.2 (b) 7.8

8.23 (a) 7.7 (b) 7.4

6.30 (a) 5.9 (b) 5.7

5.86 (a) 5.5 (b) 5.3

12.75 (a) 12.0 (b) 11.5

45.,1 (a) 43.0 (b) 41.1

100.00 (a) 94.0 (b) 90.0

Fraction (%) of Total Inci- dent Radiation Absorbed in

metres of Water. z metres of Pool Water (d) Distilled (p) Pool (a) Summer (b) Wit

z 0.5 1.0 1.5 2.0 3.0 z 0.5 1.0 1.5 2.0 3.0

(d) 1.3 2.5 3.9 5.0 7.4 (a) 0.i 0.2 0.3 0.4 0.6 (p) 1.7 3.0 4.7 6.0 8.9 (b) 0.i 0.2 0.3 0.4 0.6

(d) 0.7 1.4 2.0 2.7 4.0 (a) 0.0 0.1 0.1 0.2 0.3 (p) 0.8 1.7 2.4 3.2 4.8 (b) 0.0 0.1 0.1 0.2 0.2

(d) 0.0 1.0 1.0 i.i 1.6 (a) 0.0 0.1 0.i 0.i 0.2 (p) 0.0 1.2 1.2 1.3 1.9 (b) 0.0 0.i 0.i 0.i 0.2

(d) 0.8 1.6 2.4 3.2 4.7 (a) 0.1 0.2 0.2 0.3 0.4 (p) 1.0 1.9 2.9 3.8 5.6 (b) 0.i 0.1 0.2 0.3 0.4

(d) 3.8 7.5 ii.0 14.4 20.8 (a) 0.3 0.5 0.8 1.0 1.5 (p) 4.6 9.0 13.2 17.3 25.0 (b) 0.3 0.5 0.7 1.0 1.4

(d) 12.8 23.9 33.6 42.0 55.9 (a) 0.8 1.6 2.2 2.8 3.7 (p) 15.4 28.7 40.3 50.4 67.1 (b) 0.8 1.5 2.1 2.7 3.5

(d) 20.4 36.6 49.5 59.8 74.5 (a) 2.9 5.3 7.1 8.6 10.7 (p) 24.5 43.9 59.4 71.8 89.4 (b) 2.8 5.0 6.8 8.2 10.2

(d) 70.2 91.2 98.4 99.2 i00 (a) 36.2 43.0 (p) 84.2 i00 (b) 34.6 41.1

(a) 40 51 54 (b) 39 49 53

56 54

60 58

s~ vol. : . ~ . ~ .~ 259

260 J. L. A. FRANCEY and P. GOLDING

covers. As can be seen from Fig. 1 the covers vary in bubble shape and size and the transmissivity varies ac- cording to the path being through a bubble or between bubbles. Measured results given in Table 2 are average values for each cover, measurements being made out- doors with covers horizontal under clear sky, stable conditions.

Incident total solar radiation levels were measured using an Eppley Radiometer having an accurate cosine response to incident radiation. The pyranometer was mounted on a flat-black surfaced, horizontal comparisons table, clear of buildings and reflecting objects, with the output connected to a chart recorder having an accuracy of +- 0.15 mV, equivalent to 5 Wm -2.

A test run began with careful instrument calibration followed by a recording of the prevailing global solar flux. Then 1 m 2 of selected cover material was horizon- tally centered upon and situated 0.25 m above the hemis- pherical glass window of the pyranometer. Allowing for the response time of the pyranometer, the cover was

'=' 1 1 (c'--V 7 "

lot t J k . J

I I I I o

cm

Fig. 1. Diagramatic cross-section of the solar pool cover materials. All have air-cells, through the shape and volume vary. (A) PVC (B) peA1 and PpeA2 (C) pe A3, peC2 and peDl(D) peC3 and peE2 (E) pe D2andpeE1 (F) pe CI(G)peBI and(H) a5-cm scale for comparison.

slowly manouvered in the horizontal plane to allow for reading deviation caused by the variations in the air-cell cover material cross-sections, illustrated in Fig. 1. Hav- ing obtained a chart-recorded, repeatable variation be- tween extrema, as shown for example in Fig. 2, the cover material was removed and the prevailing global solar flux again recorded. Each cover was tested in turn, following this procedure. The percentage transmittances were than determined from the results, being the ratios of the average amount of light received by the pyranometer with and without the covers in location. As defined, the percentage transmittance is the average transparency of the solar pool cover to incident sunshine.

The polyvinylchloride (PVC) based solar pool cover was observed to have the highest average transmittance, (86 per cent) of all the covers tested, even though it was made of the heaviest gauge material. The light yellow polyethylene covers, containing titanium oxide stabil-

900

E

800

E

o _c 700

hJ B

II130 1140 - - ItlSO ----t200

Tlme, hr

Fig. 2. A typical chart-recorded variation of the global solar flux, with (A) a PVC cover and (B) a polyethylene peB! cover in situ over the pyranometer. The initial and final sections are record-

ings of the prevailing solar flux at the time (20 March 1979).

Cover Desig- nations

pvc

PeA1

peA3

peA2

peCl

peBl

peC2

peC3

peDl

peD2

peEl

peE2

Table 2. The transmittance of pool covers to sunlight

Base Material

polyvinyl- chloride

polyethylene

polyethylene

polyethylene

polyethylene/ ethylenevinyl- acetate

polyethylene

polyethylene

polyethylene

polyethylene

polyethylene

polyethylene

polyethylene

Visual Appearance

clear, purple tinge

clear, yellow tinge

light yellow

yellow

light blue

light yellow

light blue

light blue

bright blue

cobalt blue

black

black

Material Thickness

(mm)

0.44

0.14

0.09

0.12

0.09

0.11

0.i0

0.17

0.10

0.19

0.17

0.17

, J Bubble Bubble IPercentage Height Diameter Trans-

(mm) mittance (%) (ram)

4.27

7.0

6.4

7.0

5.1

4.9

6.1

5.6

6.1

6.1

7.0

5.8

46.8 86

19.1 83

16.9 81

19.1 80

12.0 79

16.9 77

16.2 77

14.1 61

16.2 60

10.0 35

I0.0 0

15.0 0

Swimming pool covers used for direct solar heating 261

izers, were also favourably transparent to solar radiation. Of the light blue polyethylene covers, the one containing a small proportion of ethylene vinyl acetate (eva) had the highest transparency. The deep blue coloured cover had an expected zero transmittance to solar radiation. These figures are in contrast to the Australian window glass used as a standard, having a transmittance of 84 per cent. The PVC cover is formed using a high frequency welding technique whereas the polyethylene covers are heat welded. The latter process has a deleterious effect, reducing the transmissive character of the plastic film during cover manufacture. Transmission is also a func- tion of the extinction coefficient of the base materials and it is observed that covers with strong absorbing pigments have inherent lower transittances to impinging solar radiation. Light in the blue part of the solar spec- trum has little chance of being absorbed in the water. Hence, although light blue covers are generally lower transmitters, successful transmission of blue wavelength radiation would have had little impact on the overall heating effect. Bright blue covers, however, broadly reduce the amount of light of all wavelengths transmitted and subsequently reduce the amount of radiation avail- able for absorption in the swimming pool water.

4. SPECTRAL SELECTIVITY AND OPACITY TO THERMAL

RADIATION.

Having determined for each cover the average fraction of the total incident sunlight that will be usefully trans- mitted to the swimming pool through the covers and knowing the amount of that fraction which will be ab- sorbed as a function of depth it is valuable to ascertain the ability of the pool cover to retard the flow of heat as thermal radiation from the water to the environment. This can be achieved by considering the selective trans- mitting capacity of the solar pool covers. Transmittance, like reflectance and absorptance, is a function of waw.qength and plastic solar pool covers may be expec- ted to transmit selectively, their apparent transmittance being a function of the wavelength of incident radiation. To be a successful selective transmitting surface, the

cover must be transparent to sunlight and make the transition from transmissive to reflective behaviour in the wavelength region between 1.5 and 3.0 #m enabling sunlight to enter the water but inhibiting the loss of thermal IR from the absorber.

The spectral selectivity of the solar pool cover materials was investigated using a Carey-17 Spectro- photometer operating in the percentage transmission mode. Modifications to the sample holder and photo detection apparatus enabled the transmissivity through the air-cells, r~,, and between air-cells, zb~, to be deter- mined independently, necessary since each have different numbers of interfaces. The transmittance cur- ves in the IR wavelength region were calibrated with transmission curves obtained using a Jasco I.R.A.I. Diffraction Grating Infra-Red Spectrophotometer. The resulting spectrographs in the wavelength region 2.5- 15/zm for the PVC cover are shown as examples in Fig. 3.

The transmission of radiation is seen to be very much wavelength dependent, with numerous strong absorption bands. The pool covers do not transmit in the UV; in general they are known to absorb it with the long term possibility of damaging the plastic. The transmission is generally high in the solar spectrum and lower and more variable in the thermal IR region. The areas under such graphs may be used to calculate the average transmittances over the two wavelength regions of in- terest, either side of 2.5 #m, taking into account the pro- portion of each cover that is and is not air-celled.

The results of such an analysis, presented in Table 3, show that the PVC cover, as well as having the highest transmittance to sunlight, has by far the lowest trans- mittance to thermal IR radiation. The polyethylene based covers have much higher transmittances to outgoing longwave radiation that would be emitted from an open- surfaced, heated swimming pool.

5. REFLECTION FROM THE COVER

The transmittance is also a function of the angle of incidence of incoming radiation. To function when on the

IO0

9O

E 8o

$ 70 o_

E 60 - o

5 o - E

4 o - o

~- s o -

2 0 -

I 0 -

0 0.2

t t I t I-fl i I . J L - , , , --.'~ 0.6 Ii0 1.4 1.8 3 5 7 9 II 115 115

Wavelength, p .m

Fig. 3. The variation of transmission as a function of wavelength for the PVC cover, (a) through the air-cells and (b) between the air cells.

262 J.L.A. FP.ANCEY and P. GOLDING

Table 3. The transmission characteristics of pool covers over the solar spectrum (r.a) and in the

Cover Desig- nation

pvc

peC1

peAl

peA2

peB1

peA3

peC2

peDl

peC3

peD2

peel

per2

Proportion of Cover that is air-cells

.64

.63

.63

.63

.62

.62

.62

.62

.59

.56

.56

.59

Volume of air inside air-cell (mm3)xlO0

47

8

20

20

ii

14

13

13

9

5

5

I0

Transmittance of cover air- cells (%)

Xa~ rb~

87 6

82 29

85 34

84 31

80 36

83 48

80 48

62 43

63 44

35 26

0 0

0 0

Propor- tion of cover between air-cells

.36

.37

.37

.37

.38

.38

.38

.38

.41

.44

.44

.41

Transmittance of cover between air- cells (%)

XaA TbA

84 5

73 27

80 31

74 29

72 34

77 46

72 46

56 39

59 38

35 24

0 0

0 0

thermal IR (%~)

Mean Transmittance of cover (%)

~aA ~bA

86 6

79 28

83 33

80 30

77 35

81 47

77 47

60 41

61 42

35 25

0 0

0 0 i

pool during the day solar pool covers require the trans- mission of incident radiation through a slab or film of material--in the case of the air-cell through two suc- cessive slabs. There are thus a series of two and four interfaces per cover to cause reflection loss.

The variation in transmission through the light blue polyethylene/eva cover with changing incident radiation angle was investigated using the Carey-17 instrument. A specimen holder which could be adjusted away from the normal of the incoming light at regular 10 ° intervals was devised and 8 spectral transmission curves obtained. The observed decrease in transmission as a function of in- creasing incidence angle is illustrated in Fig. 4, showing the transmittance decreasing markedly at high angles of incidence, mainly as a result of increased reflection losses and to a lesser extent due to absorption of radiation in the cover material.

6. SUMMARY AND CONCLUSIONS

A considerable proportion of the sunlight incident on an outdoor swimming pool is available for absorption in

lOP

9 0

8O o

~o

6o

5o

o 40

50

2O

IO

I0 20 50 40 50 60 70 80 90

Angle of incidence, degrees

Fig. 4. Transmittance of the polyethylene/eva cover as a func- tion of the angle of incidence.

the pool water. Reflection at the water surface varies with the time of day and season and accounts for a fraction of the loss of impinging radiation.

The thermal conductivity of water is low. If the water in a swimming pool was heated only by conduction from the surface, the thermal strata would be radically different. Instead the radiation successfully entering the water is differentially absorbed by the pool water. Ab- sorption is a function of the depth of water and the wavelength of radiation. The transmission is low and the extinction coefficient high in the IR. The transmission is maximal in a region about blue light and falls slowly in the violet and long UV.

Much of the heat gain is returned to the surrounding environment via heat movement and mass transfer. A significant proportion of this heat loss is returned to space through the emission of radiation from the pool surface, particularly at night.

A solar pool cover provides a simple means of main- taining elevated swimming water temperatures. The cover has a blanketing effect on losses, including significant retardation of the flow of heat as thermal radiation from the pool. The spectral selectivity of such covers can be readily evaluated from transmission data and provide a means of assessing the covers ability to perform as a selective transmitting surface.

It should be noted that these transmission measure- ments were all performed with the covers in air at room temperature. In use, a cover floats on a pool and has one surface at water temperature so that results presented here are not fully representative but they give a very clear indication of how various covers may be expected to perform in practice.

Acknowledgements--Solar pool cover materials used in the experiments were donated by Airlite Packaging Pty. Ltd., Aqualeisure, Pakrite Indt:stries Pty. Ltd., Plaspiline Industries Pty. Ltd. and Rheem Australia. We are grateful for financial support through a Commonwealth Postgraduate Scholarship. We are greatful for technical support by the Dept. of Physics, Monash University and in particular the Mechanical Workshop directed by R. Horan.

Swimming pool covers used for direct solar heating 263

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